1. Field of the Invention
This invention relates to a method of measuring light scattering efficiency of white pigments.
2. Description of the Related Art
Light scattering properties are the primary product attributes of white pigments, particularly TiO.sub.2 pigments. Precise, accurate evaluations of light scattering properties of pigments are important to guide product development, improve control of the manufacturing process, and aid consumers as they optimize use of pigments.
Two key properties in essentially all end-use applications of white pigments are (a) scattering efficiency for delivering opacity, which relates to hiding power and tint strength; and (b) undertone, which relates to the change of scattering efficiency as the wavelength of light varies. Opacity is the ability of a pigment to mask the features of the underlying substrate. Undertone of a white pigment may be described as the hue (bluish or reddish) of a gray paint containing the pigment in an admixture with a standard black pigment. Generally, a bluish undertone is characteristic of small particle size, while a reddish undertone is characteristic of larger and less uniform particle size. Undertone is particularly important in applications where color matching is needed.
Light scattering properties are typically measured on end-use products or lab simulations of end-use applications. Such measurement procedures are often complex, time-consuming and subject to errors due to variability in raw materials, variability introduced by having a multi-step process, and operator variability. Therefore, product characterization and process control have generally focused on fundamental properties, such as particle size measurements. Correlations between particle size and end-use properties are often used to infer actual optical properties of the pigments. Most particle size analyzers report particle sizes based on theoretical equations that assume a spherical shape. Therefore, the reported sizes are equivalent spherical diameters. However, pigments can have complex microstructures. For example, TiO.sub.2 pigments contain twinned and aggregated primary crystals and non-spherical particles. Further, most commercial grades of TiO.sub.2 pigments have hydrous oxide coatings. Therefore, measurements of equivalent spherical diameters may not accurately predict end-use performance of a pigment.
Pigments provide opacity or hiding power by diffuse light scattering in end-use applications. Measurements of opacity that do not include the diffuse component of transmittance do not provide results that reliably correlate with end-use properties.
Heretofore, scattering efficiency has typically been measured at a green light wavelength (540-580 nm) in the center of the visible spectrum where the eye is most sensitive. Undertone characterizes the change of scattering efficiency across the visible spectrum and therefore has been determined by comparing scattering efficiency at a blue wavelength relative to scattering efficiency at a red wavelength of light. Test methods in the prior art characterize pigments by (1) measurements on the end-use or a lab simulation of the end-use application, or (2) measurements on slurries of the pigment.
As an example of the first type, Bruehlman and Ross in J. Paint Techn. 1969, vol. 41, pp. 584-596 disclose use of transmission measurements using a spectrophotometer with an integrating sphere to determine scattering efficiency. Measurements are taken at 5500 .ANG. (550 nm) of TiO.sub.2 coating formulations deposited on plastic sheets. An improvement to the method of Bruehlman and Ross was described by Abrams, et al., in "Proceedings of 1996 TAPPI Coating Conference", TAPPI Press, Atlanta, Ga., 1996, pp. 185-192. For applications where TiO.sub.2 is used in porous media, e.g., paper coatings or paint films formulated above the critical pigment volume concentration, Abrams, et al. incorporate information on the porosity of the coating into calculations of hiding power.
As an example of the second type, Losoi in "Proceedings of 5th International Paint Congress", Sao Paulo, Brazil, Sep. 15, 1997, p. 213 (ABRAFATI, Brazilian Association of Paint Manufacturers, Rua Augusta, 2516-2.degree. andar -cj.22-01412-100-Sao Paulo-SP) describes methods for determining optical properties of TiO.sub.2 pigments dispersed in water. These pigments are aggregates of smaller primary crystals. Absorbance of a dilute slurry at 550 nm is used as a predictor of gloss properties. It also gives some indication of hiding power or tint strength, but correlations are limited unless independent information is available on the primary crystal size. A method for measuring undertone is based on the ratio of absorbance measurements of a dilute slurry of TiO.sub.2 at 400 and 700 nm.
Turnstall, U.S. Pat. No. 4,279,512 discloses a method for measuring mean particle size of particles suspended in a continuous medium which involves measuring non-scattered radiation which passes through the continuous medium at three different wavelengths, R1, R2 and R3. Attenuation ratios are determined from the measurements. The attenuation ratios are compared with a set of previously calculated attenuation ratios for a selection of mean particle sizes, corresponding to the determined attenuation ratios. Use of the method to measure mean particle size of TiO.sub.2 particles in aqueous suspension is disclosed.
Methods based on characterization of pigment slurries have major advantages over methods that require preparation of the end-use product: they are faster, less expensive, and can normally be determined with much higher precision. But prior art methods using pigment slurries have had limitations that restrict their ability to give results which directly correlate with performance in end-use applications.
Scattering performance is very sensitive to the difference between the refractive index of the pigment particle and the refractive index of the medium around the particle. The refractive index of water is low compared to the refractive index of the medium in end-use applications. Therefore measurements of optical properties of aqueous slurries of pigments in the blue-green region of the visible spectrum (450-600 nm) may provide inaccurate information regarding performance in an end-use medium.